Expansion tank

ABSTRACT

An expansion tank includes a plurality of partition walls separating a storage chamber of the tank main body into a plurality of compartment chambers, each partition wall provided with a lower communicating portion and an upper communicating portion formed in a lower part and an upper part thereof, respectively, to communicate the adjoining compartment chambers to each other, wherein the compartment chambers include an inlet chamber having an inlet, an outlet chamber having an outlet, and intermediate chambers, and wherein one of the compartment chambers other than the outlet chamber is provided with a communication port, and a lower edge of the upper communicating portion of the partition wall defining the compartment chamber provided with the communication port is lower than a lower edge of the upper communicating portion of another one of the partition walls.

TECHNICAL FIELD

The present invention relates to an expansion tank for a cooling system of an internal combustion engine.

BACKGROUND ART

An expansion tank (reserve tank) used for a cooling system of an internal combustion engine is connected to a coolant liquid circulation passage of the cooling system including a radiator or the like, and is configured to absorb the volume fluctuation of the coolant liquid owing to temperature changes by admitting and expelling the coolant liquid into and out of the expansion tank as required. In a known expansion tank, the interior of the tank is separated into a plurality of storage chambers ranging between the inlet and the outlet of the tank so that the gas phase of each storage chamber communicates with the adjacent storage chambers via upper communicating portions provided in upper parts of the associated partition walls, and the liquid phase of each storage chamber communicates with the adjacent chambers via lower communicating portions provided in lower parts of the associated partition walls. See Patent Document 1, for instance.

In such an expansion tank, gas bubbles in the coolant liquid ascend as the coolant liquid flows from one storage chamber to another through the lower communicating portions from the inlet side to the outlet side, whereby gas liquid separation is performed, and the gas bubbles are prevented from flowing into the circulation passage.

PRIOR ART DOCUMENT(S) Patent Document(s)

Document 1: JPH06-146883A

Document 2: JP2014-118884A

SUMMARY OF THE INVENTION Task to be Accomplished by the Invention

In the expansion tank described above, the gas liquid separation performance can be improved by increasing the number of storage chambers separated by the partition walls in the tank interior, but this reduces the volume of each storage chamber. If the volume of each storage chamber is reduced, the liquid level of the liquid storage chamber could rise rapidly in a corresponding manner when the inflow of the coolant liquid into the storage chamber exceeds the outflow of the coolant liquid out of the storage chamber via the lower communicating portion. As a result, the liquid level of the storage chamber may rise to such an extent that the coolant liquid may flow out of the storage chamber via a communication port (relief port) provided in an upper part of the expansion tank.

An object of the present invention is to prevent the coolant liquid from flowing out of the storage chamber via the communication port even when the inflow of the coolant liquid into the storage chamber should increase.

Means to Accomplish the Task

The expansion tank according to the present invention comprises a tank main body (12) defining a storage chamber (15) for storing a coolant liquid therein; and a plurality of partition walls (16, 18, 20, 22) separating the storage chamber into a plurality of compartment chambers (24, 26, 28, 30); wherein the partition walls (16, 18, 20, 22) include one provided with a lower communicating portion (32, 34, 36) and an upper communicating portion (62, 64, 66, 68) formed in a lower part and an upper part thereof, respectively, to communicate the adjoining compartment chambers (24, 26, 28, 30) to each other; wherein the compartment chambers include an inlet chamber (24) having an inlet (44), an outlet chamber (30) having an outlet (40), and at least one intermediate chamber (26, 28); and wherein one of the compartment chambers other than the outlet chamber (30) is provided with a communication port (52), and a lower edge of the upper communicating portion (62, 66) of the partition wall (16, 20) defining the compartment chamber (26) provided with the communication port (52) is lower than a lower edge of the upper communicating portion (64, 68) of another one of the partition walls (18, 22).

Owing to this arrangement, even when the liquid level in the compartment chamber (26) provided with the communication port (52) should rise, the coolant liquid is allowed to flow into a next compartment chamber (28) via the upper communicating portion (62, 66) so that the liquid level in the compartment chamber (26) in question is prevented from rising excessively. As a result, the liquid level in the compartment chamber (26) provided with the communication port (52) is prevented from reaching the communication port (52) so that the coolant liquid is prevented from flowing out of the tank via the communication port (52).

Preferably, in the expansion tank of the present invention, the tank main body (12) is box-shaped and has an open upper end which is closed by a top lid (14), and each upper communicating portion (62, 64, 66, 68) is defined between the corresponding partition wall (16, 18, 20, 22) and the top lid (14).

Thereby, the upper communicating portions (62, 64, 66, 68) can be formed in a simple manner.

Preferably, in the expansion tank of the present invention, at least one of the upper communicating portions (62, 64, 66, 68) consists of an opening having a lower edge defined by at least a part of an upper edge (16A, 18A, 20A, 22A) of the corresponding partition wall (16, 18, 20, 22).

Thereby, the opening area of each upper communicating portion (62, 64, 66, 68) can be maximized.

Preferably, in the expansion tank of the present invention, the at least one intermediate chamber includes a first intermediate chamber (26) and a second intermediate chamber (28), and the communication port (52) is provided in the first intermediate chamber (26) adjoining the inlet chamber (24), the first intermediate chamber (26) and the inlet chamber (24) adjoining the second intermediate chamber (28) on a same side, and wherein a part of the partition walls (16, 20) defining the first intermediate chamber (26) adjoining the partition wall defining the second intermediate chamber (24) has an upper edge (20B, 20B) higher than an upper edge (16A, 20A) of a remaining part of the partition walls.

Thereby, the coolant liquid is prevented from directly flowing from the inlet chamber (24) to the second intermediate chamber (28) over the upper edges (16A, 20A) of the partition walls (16, 20) so that the decrease in the gas liquid separation performance owing to the direct flow of the coolant liquid from the inlet chamber (24) to the second intermediate chamber (28) can be avoided.

Preferably, in the expansion tank of the present invention, the communication port (52) communicates with the first intermediate chamber (26), and an upper edge (16A) of the partition wall (16) separating the first intermediate chamber (26) from the inlet chamber (24) is higher than an upper edge (20A) of the partition wall (20) separating the first intermediate chamber (26) from one of the compartment chambers (28) other than the inlet chamber (24).

Thereby, the coolant liquid is prevented from flowing from the inlet chamber (24) to the intermediate chamber (26) and the other compartment chamber (28) over the upper edge (16A) of the partition wall (16) so that the gas liquid separation performance that could be impaired by the direct flow of the coolant liquid from the inlet chamber (24) to the intermediate chamber (26) and the other chamber (28) is prevented from being impaired.

Preferably, in the expansion tank of the present invention, an opening area of the upper communicating portion (62, 66) of the partition wall (16, 20) defining the compartment chamber (26) communicating with the communication port (52) is greater than an opening area of the lower communicating portion (32, 34) of the partition wall (16, 20) defining the compartment chamber (26) communicating with the communication port (52).

Thereby, the flow rate of the coolant liquid that flows past the upper communicating portions (62, 66) to the other compartment chambers (24, 28) is increased so that the liquid level of the coolant liquid in the compartment chamber (26) in question can be prevented from reaching the communication port (52) in an even more effective manner.

Effect of the Invention

According to the expansion tank of the present invention, when the liquid level of the compartment chamber provided with the communicating hole rises, the coolant liquid flows to the other compartment chambers through the upper communicating portion so that the liquid level in the compartment chamber provided with the communicating hole is prevented from reaching the communication port in an even more effective manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an expansion tank of a first embodiment of the present invention;

FIG. 2 is a perspective view of the expansion tank of the first embodiment with a top lid thereof removed;

FIG. 3 is a sectional view taken along line III-III of FIG. 1;

FIG. 4 is a sectional view taken along line IV-IV of FIG. 1;

FIG. 5 is a perspective view of an expansion tank of a second embodiment of the present invention with a top lid thereof removed; and

FIG. 6 is a perspective view an expansion tank of a third embodiment of the present invention with a top lid thereof removed.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

A first embodiment of an expansion tank according to the present invention is described in the following with reference to FIGS. 1 to 4. For the sake of convenience of description, various directions such as front, rear, up, down, left and right directions are defined as shown in the drawings.

The expansion tank 10 includes a box-shaped tank main body 12 having an open upper end and a top lid 14 that closes the upper opening of the tank main body 12. The tank main body 12 and the top lid 14 are made of plastic material, and joined together in an air tight manner by vibration welding or the like. The tank main body 12 has a substantially rectangular parallelepiped shape, and includes a tank front wall 12A, a tank rear wall 12B, a tank right wall 12C, a tank left wall 12D, and a tank bottom wall 12E so as to define a storage chamber 15 for coolant liquid.

In the tank main body 12, a first partition wall 16, a second partition wall 18, a third partition wall 20, and a fourth partition wall 22 are integrally molded as vertical walls standing upright from the tank bottom wall 12E. The partition walls 16, 18, 20 and 22 separate the storage chamber 15 into a plurality of compartment chambers (an inlet chamber 24, a first intermediate chamber 26, a second intermediate chamber 28 and an outlet chamber 30).

The second partition wall 18 and the third partition wall 20 are laterally aligned with each other, and extend linearly between the tank front wall 12A and the tank rear wall 12B substantially in parallel with the tank right wall 12C. In other words, the second partition wall 18 and the third partition wall 20 may be considered as a single partition wall extending between the tank front wall 12A and the tank rear wall 12B. When viewed in such a way, the front half of the single partition wall consists of the second partition wall 18 while the rear half of the single partition wall consists of the third partition wall 20, and an intermediate position of the single partition wall with respect to the fore and aft direction defines a boundary between the second partition wall 18 and the third partition wall 20.

The first partition wall 16 extends linearly from the boundary between the second partition wall 18 and the third partition wall 20 to the tank right wall 12C along an intermediate part of the tank main body 12 with respect to the fore and aft direction. Thus, the first partition wall 16 defines a shape of letter T jointly with the second partition wall 18 and the third partition wall 20.

The fourth partition wall 22 extends between the tank front wall 12A and the tank rear wall 12B in parallel with the second partition wall 18 and the third partition wall 20 on the same side of the second partition wall 18 and the third partition wall 20 as the tank left wall 12D.

As a result, the first partition wall 16 divides the right side portion of the interior of the tank main body 12 in the fore and aft direction into the inlet chamber 24 and the first intermediate chamber 26. The second partition wall 18 divides the front side portion of the interior of the tank main body 12 laterally into the inlet chamber 24 and the second intermediate chamber 28. The third partition wall 20 divides the rear side portion of the interior of the tank main body 12 laterally into the first intermediate chamber 26 and the second intermediate chamber 28. The fourth partition wall 22 divides the left side portion of the interior of the tank main body 12 laterally into the second intermediate chamber 28 and the outlet chamber 30.

A lower part of the first partition wall 16 is formed with a first lower communicating portion 32 communicating the inlet chamber 24 with the first intermediate chamber 26 adjoining the inlet chamber 24 via the first partition wall 16. A lower part of the third partition wall 20 is formed with a second lower communicating portion 34 for communicating the first intermediate chamber 26 with the second intermediate chamber 28 adjoining the first intermediate chamber 26 via the third partition wall 20. A lower part of the fourth partition wall 22 is formed with a third lower communicating portion 36 communicating the second intermediate chamber 28 with the outlet chamber 30 adjoining to the second intermediate chamber 28 via the fourth partition wall 22.

The first lower communicating portion 32 consists of an opening elongated in the fore and aft whereas the second lower communicating portion 34 and the third lower communicating portion 36 each consist of an opening elongated in the lateral direction. The opening direction of the first lower communicating portion 32 is at a 90 degree angle relative to those of the second lower communicating portion 34 and the third lower communicating portion 36. The second lower communicating portion 34 and the third lower communicating portion 36 are directed in the same direction, but are offset relative to each other in the fore and aft direction. The lower communicating portions 32, 34 are 36 are formed as rectangular communication openings each having a lower edge defined by the tank bottom wall 12E, and have a substantially same opening area.

A front left part of the bottom wall 12E of the tank main body 12 is integrally formed with an outlet nipple 38. The outlet nipple 38 defines an outlet 40 opening out at the bottom of the outlet chamber 30. To the outlet nipple 38 is connected a hose (not shown in the drawings) to connect the outlet 40 to a coolant liquid circulation passage of an internal combustion engine (not shown in the drawings).

A front right side of the top lid 14 is integrally formed with an inlet nipple 42. The inlet nipple 42 defines an inlet 44 opening out at the top of the inlet chamber 24. A hose (not shown in the drawings) to connect the inlet 44 to the coolant liquid circulation passage of the internal combustion engine (not shown in the drawings) is connected to the inlet nipple 42.

An upper rear side part of the top lid 14 is integrally formed with a cylindrical replenishing portion 46 which is provided with a supply port 48 opening out to the upper part of the first intermediate chamber 26. An air vent nipple 50 is integrally formed on a side part of the replenishing portion 46. The air vent nipple 50 defines a communication port (air relief opening) 52 communicating with the first intermediate chamber 26 via the supply port 48. To the air vent nipple 50 is connected a relief tube 51 whose free end side is bent downward.

A filler cap 54 is detachably attached to the upper end of the replenishing portion 46. The filler cap 54 includes a pressure regulating valve 60 provided with a valve member 58 biased in a valve closing direction by a spring 56. When the pressure inside the tank (tank internal pressure) is less than a predetermined value, the pressure regulating valve 60 closes, and interrupts the communication between the communication port 52 and the first intermediate chamber 26. When the pressure inside the tank is equal to or higher than the predetermined value, the pressure regulating valve 60 opens, and establishes communication between the communication port 52 and the first intermediate chamber 26.

The top lid 14 defines a first upper communicating portion 62 (see FIG. 4) that communicates the inlet chamber 24 with the first intermediate chamber 26, in cooperation with an upper edge 16A of the first partition wall 16, a second upper communicating portion 64 (see FIG. 4) that communicates the inlet chamber 24 with the second intermediate chamber 28, in cooperation with an upper edge 18A of the second partition wall 18, a third upper communicating portion 66 (see FIG. 3) that communicates the first intermediate chamber 26 with the second intermediate chamber 28, in cooperation with an upper edge 22A of the fourth partition wall 22, and a fourth upper communicating portion 68 (see FIG. 3) that communicates the second intermediate chamber 28 with the outlet chamber 30. In other words, these upper communicating portions 62, 64, 66 and 68 are formed by openings located above the partition walls 16, 18, 20 and 22, respectively, and having lower edges thereof defined by the upper edges 16A, 18A, 20A, 22A of the partition walls 16, 18, 20, 22, respectively. The opening areas of the upper communicating portions 62, 64, 66 and 68 may differ from one another, but are all larger than the opening area of the lower communicating portions 32, 34 and 36.

The upper edges 16A and 20A of the first partition wall 16 and the third partition wall 20 defining the first intermediate chamber 26 are lower than the upper edges 18A and 22A of the second partition wall 18 and the fourth partition wall 22 by a dimension H. In other words, the lower edges of the first upper communicating portion 62 and the third upper communicating portion 66 are lower than the lower edges of the second upper communicating portion 64 and the fourth upper communicating portion 68. The opening areas of the upper communicating portions 62, 64, 66 and 68 may differ from one another, but are larger than the opening areas of the lower communicating portions 32, 34 and 36.

A plurality of ribs 70, 72, 74 and 76 are integrally formed on the lower part (bottom part) of the top lid 14. As shown in FIG. 4, the rib 76 is positioned in front of the inlet 44 so that the coolant liquid flowing into the inlet chamber 24 from the inlet 44 impinges on the rib 76. This prevents the coolant liquid from the inlet 44 from flowing directly into the first intermediate chamber 26 through the first upper communicating portion 62.

The tank front wall 12A is formed with a shelf portion 13 in an upper part of the inlet chamber 24. The shelf portion 13 shortens the falling distance of the coolant liquid flowing into the inlet chamber 24 from the inlet 44 so that the foaming of the coolant liquid due to a fall can be minimized.

In the normal condition where the flow rate of the coolant liquid flowing into the inlet chamber 24 from the inlet 44 is steady and lower than a prescribe value, the coolant liquid that has flowed into the inlet chamber 24 from the inlet 44 passes into the first intermediate chamber 26 via the first lower communicating portion 32, and into the second intermediate chamber 28 via the second lower communicating portion 34. The coolant liquid then flows into the outlet chamber 30 via the third lower communicating portion 36 before being expelled out of the tank 10 via the outlet 40. In this steady state, even if the liquid level of the coolant liquid in the inlet chamber 24, the first intermediate chamber 26, the second intermediate chamber 28, and the outlet chamber 30 rises substantially in the same manner, and the liquid level of the coolant liquid does not rise above the upper edge 16A of the first partition wall 16 or the upper edge 20A of the third partition wall 20. As a result, the entire gas phase in the upper part of the tank is kept in communication via the upper communicating portions 62, 64, 66 and 68, and the coolant liquid does not flow past the upper edges 16A, 18A, 20A and 22A of the partition walls 16, 18, 20 and 22.

Therefore, as the coolant liquid flows from the inlet chamber 24 to the outlet chamber 30 via the first intermediate chamber 26 and the second intermediate chamber 28, the gas phase is separated from the liquid phase as rising bubbles so that the coolant liquid freed from gas bubbles is returned to the coolant liquid circulation passage.

When the pressure in the gas phase rises due to the rise of the liquid level, the pressure regulating valve 60 opens, and the air in the tank is released via the communication port 52. As a result, an increase in the tank internal pressure is controlled.

When the flow rate of the coolant liquid flowing into the inlet chamber 24 from the inlet 44 exceeds a predetermined value, the liquid level of the inlet chamber 24 rises due to the coolant liquid flowing into the inlet chamber 24 from the inlet 44. When the liquid level goes above the upper edge 16A of the first partition wall 16, a part of the coolant liquid flowing into the inlet chamber 24 flows over the upper edge 16A of the first partition wall 16, or, in other words, flows into the first intermediate chamber 26 via the upper communicating portion 62. This causes the liquid level of the coolant liquid in the first intermediate chamber 26 to rise rapidly so that the liquid level in the first intermediate chamber 26 becomes higher than the upper edge 20A of the third partition wall 20. As a result, a part of the coolant liquid flowing into the first intermediate chamber 26 flows over the upper edge 20A of the third partition wall 20 and into the second intermediate chamber 28. This means that the coolant liquid in the first intermediate chamber 26 flows into the second intermediate chamber 28 not only via the second lower communicating portion 34 but also via the third upper communicating portion 66 with the result that the rise of the liquid level in the intermediate chamber 26 is controlled. This is particularly the case when the opening area of the third upper communicating portion 66 is larger than the opening area of the second lower communicating portion 34.

Thereby, the liquid level of the coolant liquid in the first intermediate chamber 26 does not become significantly higher than the upper edge 20A of the third partition wall 20, and the liquid level of the coolant liquid in the first intermediate chamber 26 does not reach the communication port 52 which is located above the upper edge 20A of the third partition wall 20. By suitably selecting the opening areas of the first upper communicating portion 62 and the third upper communicating portion 66 for a given anticipated maximum flow rate, it can be arranged such that the coolant liquid is prevented from flowing out of the tank 10 via the communication port 52 without regard to the flow rate of the coolant liquid flowing into the inlet chamber 24.

Since the upper edge 18A of the second partition wall 18 and the upper edge 22A of the fourth partition wall 22 are located higher than the upper edge 16A of the first partition wall 16 and the upper edge 20A of the third partition wall 20, the overflowing of the coolant liquid over the upper edge 18A of the partition wall 18 and the upper edge 22A of the fourth partition wall 22 is less likely to occur than the overflowing of the coolant liquid over of the upper edge 16A of the first partition wall 16 and the upper edge 20A of the third partition wall 20. It means that the direct flow of the coolant liquid from the inlet chamber 24 to the second intermediate chamber 28 and the direct flow of the coolant liquid from the first intermediate chamber 26 to the second intermediate chamber 28 via an upper part of the second intermediate chamber 28 are not likely to happen. Thus, the reduction in the gas liquid separation performance of the tank 10 due to overflowing of the coolant liquid can be minimized.

When the filler cap 54 is removed, and coolant liquid is replenished excessively into the tank via the replenishing portion 46, the liquid level of the first intermediate chamber 26 rises rapidly. In such a case, the coolant liquid flows from the first intermediate chamber 26 into the inlet chamber 24 over the upper edge 16A of the third partition wall 20, and into the second intermediate chamber 28 over the upper edge 20A of the third partition wall 20 so that the excessive rise of the liquid level in the first intermediate chamber 26 can be avoided, and hence the coolant liquid is prevented from flowing out of the tank from the replenishing portion 46.

A second embodiment of an expansion tank according to the present invention is described in the following with reference to FIG. 5. In FIG. 5, the parts corresponding to those in FIG. 2 are denoted with like numerals without necessarily repeating the description of such parts.

In the second embodiment, a part of the upper edge 20B of the third partition wall 20 on the side of the inlet chamber 24, or, in other words, on the side of the boundary with the second partition wall 18 (front side) is located higher than the upper edge 20A of the third partition wall 20 on the side of the tank rear wall 12B.

According to this arrangement, the coolant liquid is less likely to flow from the inlet chamber 24 directly to the second intermediate chamber 28 over the upper edge 16A of the first partition wall 16 and the upper edge 20A of the third partition wall 20. As a result, the reduction in the gas liquid separation performance of the tank that could be caused by the direct flow of the coolant liquid from the inlet chamber 24 to the second intermediate chamber 28 can be avoided.

This action can also be obtained by positioning a part 16B of the upper edge 16A of the first partition wall 16 adjacent to the junction between the second partition wall 18 and the third partition wall 20 higher than the remaining part of the upper edge 16A of the first partition wall 16 as shown by imaginary lines in FIG. 5.

A third embodiment of an expansion tank according to the present invention is described in the following with reference to FIG. 6. In FIG. 6, the parts corresponding to those in FIG. 2 are denoted with like numerals without necessarily repeating the description of such parts.

In the third embodiment, the upper edge 16A of the first partition wall 16 and the upper edge 20A of the third partition wall 20 are both positioned lower than the upper edge 18A of the second partition wall 18 and the upper edge 22A of the fourth partition wall 22. However, the upper edge 16A of the first partition wall 16 is higher than the upper edge 20A of the third partition wall 20.

According to this arrangement, the likelihood of the coolant liquid flowing directly from the inlet chamber 24 to the second intermediate chamber 28 over the upper edge 16A of the first partition wall 16 and the upper edge 20A of the third partition wall 20 can be reduced. Also, the likelihood of the coolant liquid flowing directly from the chamber 24 to the first intermediate chamber 26 over the upper edge 16A of the first partition wall 16 can be reduced.

Thereby, the gas liquid separation performance of the tank which could be impaired by the direct flow of the coolant liquid from the inlet chamber 24 to the first intermediate chamber 26 and the second intermediate chamber 28 can be avoided.

Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to those skilled in the art that the present invention is not limited by such embodiments, but can be modified without departing from the spirit of the present invention. For instance, only the lower edge of the third upper communicating portion 66 may be lower than the lower edges of the other upper communicating portions 62, 64 and 68. The upper communicating portions 62, 64, 66 and 68 may also be formed by communicating holes formed in the upper parts of the respective partition walls 16, 18, 20 and 22.

The various components shown in the above embodiments are not necessarily entirely essential, and can be appropriately selected and substituted without departing from the spirit of the present invention.

GLOSSARY OF TERMS 10 expansion tank 12 tank main body 14 top lid 15 storage chamber 16 first partition wall 16 second partition wall 16A upper edge 16B upper edge 18 second partition wall 18A upper edge 20 third partition wall 20A upper edge 20B upper edge 22 fourth partition wall 22A upper edge 22A partition wall 24 inlet chamber 26 first intermediate chamber 28 second intermediate chamber 30 outlet chamber 32 first lower communicating portion 34 second lower communicating portion 36 third lower communicating portion 40 outlet 44 inlet 46 replenishing portion 48 supply port 52 communication port 54 filler cap 60 pressure regulating valve 62 first upper communicating portion 64 second upper communicating portion 66 third upper communicating portion 68 fourth upper communicating portion 

1. An expansion tank, comprising: a tank main body defining a storage chamber for storing a coolant liquid therein; and a plurality of partition walls separating the storage chamber into a plurality of compartment chambers; wherein the partition walls include one provided with a lower communicating portion and an upper communicating portion formed in a lower part and an upper part thereof, respectively, to communicate the adjoining compartment chambers to each other; wherein the compartment chambers include an inlet chamber having an inlet, an outlet chamber having an outlet, and at least one intermediate chamber; and wherein one of the compartment chambers other than the outlet chamber is provided with a communication port, and a lower edge of the upper communicating portion of the partition wall defining the compartment chamber provided with the communication port is lower than a lower edge of the upper communicating portion of another one of the partition walls.
 2. The expansion tank according to claim 1, wherein the tank main body is box-shaped and has an open upper end which is closed by a top lid, and each upper communicating portion is defined between the corresponding partition wall and the top lid.
 3. The expansion tank according to claim 1, wherein at least one of the upper communicating portions consists of an opening having a lower edge defined by at least a part of an upper edge of the corresponding partition wall.
 4. The expansion tank according to claim 2, wherein the at least one intermediate chamber includes a first intermediate chamber and a second intermediate chamber, and the communication port is provided in the first intermediate chamber adjoining the inlet chamber, the first intermediate chamber and the inlet chamber adjoining the second intermediate chamber on a same side, and wherein a part of the partition walls defining the first intermediate chamber adjoining the partition wall defining the second intermediate chamber has an upper edge higher than an upper edge of a remaining part of the partition walls.
 5. The expansion tank according to claim 4, wherein the communication port communicates with the first intermediate chamber, and an upper edge of the partition wall separating the first intermediate chamber from the inlet chamber is higher than an upper edge of the partition wall separating the first intermediate chamber from one of the compartment chambers other than the inlet chamber.
 6. The expansion tank according to claim 1, wherein an opening area of the upper communicating portion of the partition wall defining the compartment chamber communicating with the communication port is greater than an opening area of the lower communicating portion of the partition wall defining the compartment chamber communicating with the communication port. 